01 - Preheat oven to 350°F. Lightly grease a nonstick donut pan to prevent sticking.
02 - In a large mixing bowl, combine mashed bananas, milk, eggs, vegetable oil, granulated sugar, and vanilla extract. Whisk until the mixture is smooth and homogeneous.
03 - In a separate bowl, sift together all-purpose flour, baking powder, baking soda, ground cinnamon, and salt.
04 - Gently fold the dry ingredients into the wet mixture using a spatula until just combined. Avoid overmixing to maintain a tender crumb.
05 - Transfer batter to a piping bag or a resealable plastic bag with a snipped corner. Pipe batter into the prepared donut pan, filling each cavity about three-quarters full.
06 - Bake in the preheated oven for 12 to 15 minutes or until the tops are golden and spring back when gently pressed.
07 - Remove pan from oven and let donuts cool in the pan for 5 minutes. Carefully transfer donuts to a wire rack and allow to cool completely.
08 - For the glaze, whisk together powdered sugar, milk, and vanilla extract until smooth. Dip each cooled donut into the glaze and set aside for a few minutes to allow glaze to set.